METHOD OF PRODUCTION OF SUSTAINABLE STEEL / CARBON STEEL SHEETS THAT HAVE VERY GOOD BUILDING AND RESISTANCE CHARACTERISTICS
EXTENSION AND AN EXCELLENT UNIFORMITY
The present invention relates to the manufacture of an austenitic sheet of iron / carbon / manganese hot-rolled and cold-rolled having very good mechanical properties and, in particular, a very advantageous combination of mechanical strength and elongation at break, together with an excellent homogeneity of the mechanical properties. In the field of automotive, the continuous increase in the level of equipment of the vehicles makes it even more necessary to lighten the own structural metal. To do this, each function has to be reconsidered to improve its performance and reduce its weight. Up to now, several steel families have been created to satisfy these increasing requirements: in chronological order, for example, high-yield steels hardened by a fine precipitation of niobium, vanadium or titanium can be mentioned; steels with double phase structures (ferrite containing up to 25% martensite); and TRI P steels (transformation-induced plasticity) ferrite, martensite and austenite compounds that can be transformed with deformation. For each type of structure, the tensile strength and the deformability are rival properties, so that it is generally not possible.
obtain very high values for one of the properties if n drastically reduce the other. Thus, in the case of TRI P steels, it is difficult to obtain a resistance higher than 900 M Pa simultaneously with an elongation greater than 25%. It is also possible to mention steels having a bainitic or martensitic-bainitic structure, whose strength can be up to 1200 M Pa when hot rolled, but whose elongation is only about 10%. Although these properties may be satisfactory for various applications, they remain insufficient if additional lightening is desired by the simultaneous combination of high strength and greater ability for subsequent deformation operations and for energy absorption. In the case of the hot-rolled sheet, that is to say a sheet with a thickness ranging from about 1 to 10 mm, these properties are beneficially used to lighten ground connection pieces, wheels, reinforcement parts such as anti-roll bars. -Intrusion of doors, or parts intended for heavy vehicles (trucks, buses, etc.). In the case of the lamina lami anything cold (with a thickness that varies from approximately 0.2 mm to 6 mm), the applications are for the manufacture of parts used to achieve safety and durability in motor vehicles, or even external parts . Steels with an austenitic structure are known to meet these requirements for simultaneous strength / ductility.
as Fe-C-M n steels comprising up to 1.5% C and from 15 to 35% M n (the contents being expressed by weight) and which possibly contain other elements such as silicon, aluminum or chromium. At a given temperature, the mode of deformation of the austenitic steels depends solely on the stacking defect energy or SFE, the physical amount of which depends only on the composition and the temperature. When the SFE is reduced, the deformation proceeds consecutively from a sliding dislocation mode, to a twinning mode and finally to a martensitic transformation mode. Among these modes, the mechanical maclaje makes it possible to achieve a high mechanical hardenability: twins, which act as an obstacle to the propagation of dislocations, helping to increase the elastic limit. The SFE increases in particular with the content of carbon and manganese. In this way, austenitic steels of Fe-0.6% of C-22% of M n capable of deforming by twinning are known. Depending on the granulometry, these steel compositions produce limit values of tensile strength ranging from about 900-1,150 M Pa in combination with an elongation at break ranging from 50 to 80%. However, there is still an unmet need for a hot-rolled or cold-rolled steel sheet with a strength significantly greater than 1 150 M Pa and which at the same time has a good deformability, this being achieved without the
addition of expensive alloys. It is desired to have a steel sheet that exhibits a very homogeneous behavior during subsequent mechanical stresses. Therefore, the object of the invention is to provide a sheet or product of hot-rolled or cold-rolled steel of cheap manufacture, having a strength of at least 1200 MPa, or even of 1400 M Pa, in combination with an elongation such that the product P: strength (in M Pa) x elongation at break (in%) is greater than 60,000 or 50,000 MPa% at the level of resistance mentioned above, respectively, very homogeneous mechanical properties during the subsequent deformation or mechanical stress and a structure without martensite at any point during or after the cold deformation of this sheet or product. For this purpose, the object of the invention is an austenitic sheet of iron / carbon / manganese hot rolled, whose strength is greater than 1200 M Pa, whose product P (strength (in M Pa) x elongation at break (in %)) is greater than 65, 000 M Pa% and whose nominal chemical composition comprises, expressing the contents by weight: 0.85% < C < 1.05%; 16% < M n < 19%; Yes < 2%; To < 0.050%; S < 0.030%, P < 0.050%; N < 0.1%; and, optionally, one or more elements chosen from: Cr < 1 %; M o < 1 .50% »; Ni < 1 %; Cu < 5%; You < 0.50%; Nb < 0.50%; V < 0.50%; the rest of the composition consisting of iron and unavoidable impurities from the processing, the fraction of the recrystallized surface of the steel being equal to 100%), the fraction being
of surface of crystallized carbides of steel equal to 0% and the average grain size of the steel being less than or equal to 10 microns. The object of the invention is also an austenitic sheet of iron / carbon / manganese cold-rolled and annealed, whose strength is greater than 1200 MPa, whose product P
(resistance (in MPa) x elongation at break (in%)) is greater than 65
000 MPa and whose nominal chemical composition comprises, expressing the contents by weight 085% < C < 1 05%, 16% < Mn < 19% o, Si < 2%, Al < 0050%, S < 0030%), P < 0050%), N < 0 1%, and optionally one or more elements chosen from Cr < 1%, Mo <
1 50%, Ni < 1%, Cu < 5%, Ti < 050%, Nb < 050%, V < 050%, the rest of the composition consisting of iron and unavoidable impurities from the processing, the fraction of the surface recrystallized from the steel being equal to 100% and the average particle size of the steel being less than 5 microns. The object of the invention is also a austenitic cold-rolled and annealed steel sheet whose strength is greater than 1250 MPa, whose product P (strength (in MPa) x elongation at break (in%)) is greater than 65,000 MPa%, characterized in that the average particle size of the steel is less than 3 microns According to a preferred feature, at any point of the austenitic steel sheet, the local LC carbon content of the steel and the local manganese content MnL, expressed by weight, are such that% Mn + 97% CL > 21 66 Preferably, the nominal silicon content of the steel is
less than or equal to 0.6%. According to a preferred embodiment, the nominal nitrogen content of the steel is less than or equal to 0.050%). Also preferably, the nominal aluminum content of the steel is less equal to 0.030%. According to a preferred embodiment, the nominal phosphorus content of the steel is less than or equal to 0.040%. The object of the invention is also a process for manufacturing an austenitic sheet of iron / carbon / manganese hot rolled, whose strength is greater than 1200 MPa, whose product P (strength (in MPa) x elongation at break (in% )) is greater than 65,000 MPa%, a steel being melted in said process, whose nominal composition comprises, expressing the contents by weight: 0.85% < C < 1.05%; 16% < Mn < 19%; Yes < 2%; To < 0.050%; S < 0.030%; P < 0.050%; N < 0.1%; and, optionally, one or more elements chosen from: Cr < 1%; Mo < 1.50%; Ni < 1%; Cu < 5%;
You < 0.50%; Nb < 0.50%; V < 0.50%; the rest of the composition consisting of iron and unavoidable impurities from the process, in which a semi-finished product is poured from this steel; the semi-finished product of the steel composition is heated to a temperature between 1100 and 1300 ° C; the semi-finished product is rolled to a rolling end temperature of 900 ° C or higher; - if necessary, a maintenance time is saved
in such a way that the recrystallized surface fraction of the steel is equal to 100%; the sheet is cooled at a rate of 20 ° C / s or higher; and - the sheet is rolled at a temperature of 400 ° C or less. The object of the invention is also a process for manufacturing a sheet of austenitic hot-rolled steel, whose strength is greater than 1400 M Pa, whose product P (strength (in M Pa) x elongation at break (in%)) is greater of 50,000 M Pa%, characterized in that the sheet, hot-rolled, cooled after being rolled and unwound, undergoes cold deformation with an equivalent deformation ratio of at least 13% and less than or equal to 17%. The object of the invention is also a process for manufacturing an austenitic sheet of iron / carbon / manganese cold-rolled and annealed, whose strength is greater than 1250 M Pa, whose product P (strength (in M Pa) x elongation a breakage (in%)) is greater than 60,000 M Pa%, characterized in that a hot laminated sheet obtained by the previous process is obtained; at least one cycle is carried out, each cycle consisting of the cold rolling of the sheet in one or more successive steps and the performance of a recrystallization annealing treatment, and the austenitic average granulometry before the last cold rolling cycle followed by A recrystallization annealing treatment is less than 15 microns.
The object of the invention is also a process for manufacturing a sheet of austenitic iron / carbon / manganese steel cold rolled, whose strength is greater than 1400 M Pa and whose product P (strength (in M Pa) x elongation at break (in%)) is greater than 50,000 MPa%; characterized in that the sheet, after the final recrystallization annealing treatment, is subjected to a cold deformation with an equivalent deformation ratio greater than or equal to 6% and less than or equal to 17%. The object of the invention is also a process for manufacturing a sheet of austenitic iron / carbon / manganese cold-rolled steel, whose strength is greater than 1400 M Pa and whose product P (strength (in MPa) x elongation at break (in %)) is greater than 50,000 M Pa, characterized in that a cold-rolled and annealed sheet according to the invention is provided and the sheet is subjected to a cold deformation with an equivalent deformation ratio greater than or equal to 6%. and less than or equal to 17%). The object of the invention is also a process for manufacturing an austenitic steel sheet, characterized in that the conditions in which said semi-finished product is poured or overheated, such as the molding temperature of said semi-finished product, the welding of the liquid metal by electromagnetic forces and reheating conditions that lead to the homogenization of carbon and manganese content by diffusion, are chosen so that, in any
point of the sheet, the local content of carbon C and the local content of manganese MnL, expressed by weight, are such that:% Mn + 9.7% CL > 21.66. According to a preferred embodiment, the semi-finished product is poured in the form of a slab or poured as a thin strip between steel rollers rotating in opposite directions. The object of the invention is also the use of an austenitic steel sheet for the manufacture of structural or reinforcement elements or external parts in the field of automotive. The object of the invention is also the use of an austenitic steel sheet manufactured by means of a process described above, for the manufacture of structural or reinforcement elements or external parts in the automotive field. Other features and advantages of the invention will be apparent throughout the description presented below, provided by way of example and with reference to the attached figure 1, which shows the theoretical variation of stacking defect energy at room temperature (300 K) depending on the content of carbon and manganese. After many tests, the inventors have shown that the various requirements indicated above were met by observing the following conditions: as regards the chemical composition of the steel, carbon plays a very important role in the formation of the microstructure and properties mechanical obtained. Along with a content of
manganese ranging from 16 to 19% or by weight, a nominal carbon content greater than 0.85% makes it possible to obtain a stable austenitic structure. However, for a nominal carbon content above 1.05%, it becomes difficult to prevent the precipitation of carbides that takes place during certain thermal cycles in industrial manufacturing, in particular when the steel is cooling in the winding, degrading said precipitation the ductility and tenacity. In addition, the increase in carbon content reduces weldability. Manganese is also an essential element to increase resistance, increase the stacking defect energy and stabilize the austenitic phase. If its nominal content is less than 16%, there is a risk, as will be seen below, of the formation of a martensitic phase, which greatly reduces the deformability. In addition, when the nominal manganese content is greater than 19%, the maclage deformation mode is less favored than the perfect slip dislocation mode. Furthermore, for reasons of cost, it is not desirable that the manganese content be high. Aluminum is a particularly effective element for deoxidizing steel. Like carbon, the energy of stacking effect increases. However, aluminum is a drawback if it is present in excess in steels having a high manganese content. This is because manganese increases the solubility of nitrogen in liquid iron and, if
present an excessively large amount of aluminum in the steel, nitrogen, which is combined with aluminum, precipitates in the form of aluminum nitrides which prevent the migration of the intergranular joints during hot processing and greatly increase the risk of cracks appear A nominal content of Al of 0.050% or less prevents the precipitation of AIN. Correspondingly, the nominal nitrogen content should be 0.1% or less to prevent this precipitation and the formation of volume defects during solidification. This risk is particularly reduced when the nominal aluminum content is less than 0.030% and when the nominal nitrogen content is less than 0.050%. Silicon is also an effective element for deoxidizing steel and also for solid phase hardening. However, above a nominal content of 2%, it reduces the elongation and tends to form undesirable oxides during certain assembly processes and, therefore, must be kept below this limit. This phenomenon is greatly reduced when the nominal silicon content is less than 0.6%. Sulfur and phosphorus are impurities that weaken intergranular joints. Their respective nominal contents should not exceed 0.030% and 0.050% or respectively to maintain a sufficient hot ductility. When the nominal content of phosphorus is less than 0.040%, the risk of fragility is particularly reduced.
Optionally, chromium can be used to increase the strength of the steel by hardening in solid solution. However, since chrome reduces the stacking defect energy, its nominal content should not exceed 1%. Nickel increases stacking defect energy and contributes to high elongation at break; however, it is also desirable, for reasons of cost, to limit the nominal nickel content to a maximum of 1% or less. For similar reasons, molybdenum can also be used, this element further retarding the precipitation of carbides. For reasons of efficiency and cost, it is desirable to limit its nominal content to 1.5% and preferably to 0.4%). Similarly, optionally, a copper addition to a nominal content of not more than 5% is a way of hardening the steel by precipitation of copper metal. However, above this content, copper is responsible for the appearance of defects in the surface of the hot rolled sheet. Titanium, niobium and vanadium are also elements that can optionally be used to achieve the precipitation hardening of carbonitrides. However, when the nominal content of Nb or V or Ti is greater than 0.50%, an excessive carbonitride precipitation can produce a reduction in ductility and stretchability, which should be avoided. The method for implementing the manufacturing process according to the invention is as follows. A steel that has the
The aforementioned composition is melted. After this melting, the steel can be molded in the shape of an ingot or can be molded continuously in the form of a slab with a thickness of approximately 200 mm. The steel can also be molded into a thin slab, with a thickness of a few tens of millimeters, or in the form of a thin strip between steel rollers that rotate in opposite directions Of course, although the present description illustrates the application of the invention to flat products, it can be applied in the same way to the manufacture of long products made of Fe-CM steel n These semifinished products are molded in the first place at a temperature between 1 100 and 1300 ° C. This has the objective of making all the points reach favorable temperature intervals for the large deformations that During the rolling process, however, the temperature should not exceed 1300 ° C for fear of being too close to the solids temperature, which could be reached in any zone where manganese and / or carbon is secreted, and to produce a local onset of a liquid state that would be injurious to hot forming In the case of direct molding of the thin strip between rollers that rotate in opposite directions, the hot rolling step of these semi-finished products that starts between 1300 and 1 100 ° C can be carried out directly after molding, so that in this case a step of intermediate reheating
The conditions of production of the semi-finished product (molding, reheating) have a direct influence on the possible segregation of carbon and manganese - this point will be discussed in more detail below. The semi-finished product is hot-rolled, for example, up to a thickness of hot-rolled strip of a few millimeters. The low aluminum content of the steel according to the invention prevents excessive precipitation of AI N, which would impair the hot deformability during rolling. To avoid any cracking problem through lack of ductility, the rolling end temperature should be 900 ° C or higher. The inventors have shown that the ductility properties of the sheet obtained were reduced when the recrystallized surface fraction of the steel was less than 100% > . Accordingly, if the hot rolling conditions have not resulted in a complete recrystallization of the austenite, the inventors have shown that, after the hot rolling phase, a holding time must be saved in such a way that the fraction of recrystallized surface is equal to 100% > . In this way, this isothermal high temperature soaking phase after rolling produces a complete recrystallization. For hot-rolled sheet, it has also been shown that it is necessary to prevent carbide from precipitating (essentially cementite (Fe, Mn) 3C and perlite), which would result in deterioration
of the mechanical properties, in particular a reduction of the ductility and an increase of the elastic limit. For this purpose, the inventors have discovered that a cooling rate after the rolling phase (or after the optional holding time necessary for recrystallization) of 20 ° C / sec or more completely prevents this precipitation. This cooling phase is continued by a winding operation. It has also been shown that the winding temperature must be below 400 ° C, again to avoid precipitation. For the steel compositions according to the invention, the inventors have shown that particularly good strength and elongation to break properties are obtained when the average austenitic particle size is equal to 10 microns or less. Under these conditions, the tensile strength of the hot rolled sheet obtained in this way is greater than 1200 M Pa and the product P (strength x elongation at break) is greater than 65., 000 M Pa%. There are applications in which it is desirable to obtain even greater strength characteristics in a hot rolled sheet, with a level of 1400 M Pa or higher. The inventors have shown that these characteristics were obtained by subjecting the above hot-rolled steel sheet to a cold deformation with an equivalent deformation ratio of at least 13% and less than or equal to 17%. Therefore, this cold deformation is conferred to a sheet that
it has cooled after winding, unwinding and normally pickling. This deformation with a relatively low ratio results in the manufacture of a reduced anisotropy product without affecting the subsequent processing. In this way, although the process includes a cold deformation step, the manufactured sheet can be called a "hot rolled" sheet, in that the cold deformation ratio is extremely small compared to the usual relationships produced. during cold rolling before annealing, in order to manufacture a thin sheet, and to the extent that the thickness of the sheet thus manufactured is in the range of usual thicknesses of the hot rolled sheets. However, when the equivalent cold deformation ratio is greater than 17%, the reduction in elongation becomes such that the parameter P (resistance Rm x elongation at break A) can not reach 50,000 M Pa%. Under the conditions of the invention, despite having a very high resistance value, the sheet retains a good elongation capacity since the product P of the sheet obtained in this way is greater than or equal to 50,000 M Pa%. In the case of a cold rolled and annealed sheet, the inventors have also shown that the structure must be completely recrystallized after annealing to achieve the desired properties. Simultaneously, when the average granulometry is less than 5 microns, the resistance exceeds
1200 M Pa and the product P is greater than 65,000 M Pa%. When the average granulometry obtained after annealing is less than 3 microns, the resistance exceeds 1250 M Pa, the product P being even higher than 65,000 M Pa%. The inventors have also discovered a process for manufacturing a cold rolled and annealed steel sheet with a strength greater than 1250 M Pa and a product P greater than 60,000 M Pa%, supplying a hot rolled sheet according to the process described above. and then performing at least one cycle, where each cycle consists of the following stages: cold rolling of one or more successive steps; and recrystallization annealing, the average austenitic granulometry being before the last cold rolling cycle, subjected to recrystallization annealing, less than 15 microns. It may be desirable to obtain a cold rolled sheet with an even higher strength, greater than 1400 M Pa. The inventors have shown that these properties could be achieved by providing a cold rolled sheet having the characteristics according to the invention described above or providing a cold rolled sheet obtained using the process according to the invention described above. The inventors have discovered that the application of a cold deformation to said sheet with an equivalent deformation ratio greater than or equal to 6% and less than or equal to 17% makes it possible
achieve a resistance greater than 1400 M Pa and a product P greater than 50, 000 M Pa%. When the equivalent cold deformation ratio is greater than 17%, the reduction in elongation becomes such that the P parameter can not reach 50,000 M Pa%. The particularly important role played by carbon and manganese within the context of the present invention will be explained in detail below. To do this, reference will be made to FIG. 1, which shows, in a carbon-manganese graph (the rest being iron), the calculated stacking defect isoenergy curves, whose values vary from 5 to 30 mJ / m2. At any given deformation temperature and for any given granulometry, the deformation mode is theoretically identical for any Fe-C-Mn alloy having the same SFE. The start region of martensite is also represented in this graph. The inventors have shown that it is necessary, to appreciate the mechanical behavior, consider not only the nominal chemical composition of the alloy, for example its nominal content or medium of carbon and manganese, but also its local content. This is because it is known that, during the production of steel, the solidification causes certain elements to segregate in a greater or lesser amount. This arises from the fact that the solubility of an element within the solid phase is different from the solubility in the liquid phase. In this way, solid cores will often be produced, whose solute content is below the nominal composition, implying the final phase of the
solidification a residual liquid phase enriched in solutes. This primary solidification structure can adopt various morphologies (for example, a dendritic or equiaxial morphology) and can be pronounced to a greater or lesser extent. Although these characteristics are modified by lamination and subsequent thermal treatment, the analysis of the local elemental content indicates a fluctuation around a value that corresponds to the average or nominal content of this element. In this document it is understood that the term "local content" means the content measured by means of a device such as an electron probe. A linear or superficial exploration by means of said device allows to determine the variation in the local content. In this way, the variation in local content of an Fe-C-M n alloy, whose nominal composition is C = 0.23% > , M n = 24%, Si = 0.203%, N = 0.001%. The inventors have demonstrated a cosegregation of carbon and manganese - the zones locally enriched in carbon (or depleted in carbon) also correspond to the zones enriched in manganese (or depleted in manganese). Figure 1 shows each measured point that has a local concentration of carbon (CL) and a local concentration of manganese (Mn), forming a combination that represents the local variation of carbon and manganese in the steel sheet. , centered on the nominal content (C = 0.23% o / M n = 24%). In this case, it can be seen
that the variation in the local content of carbon and manganese is manifested by a variation in stacking defect energy, since this value varies from 7 mJ / m2 for the less rich areas in C and in M n to approximately 20 mJ / m2 for the richest areas. In addition, it is known that maclage is produced as a mode of preferential deformation at room temperature when the SFE is about 15-30 mJ / m2. In the previous case, this mode of preferential deformation may not be present at all in any part of the steel sheet and certain particular areas may have a mechanical behavior different from that expected for a steel sheet of nominal composition, in particular a lower deformability by maclaje inside certain grains. More generally it is considered that, under very particular conditions that depend, for example, on the deformation or the temperature of stress, on the granulometry, the local content of carbon and manganese can be reduced to the point of locally producing a martensitic transformation induced by deformation . The inventors have sought the particular conditions to obtain very good mechanical properties simultaneously with a great homogeneity of these properties within a sheet of steel. As explained above, the combination of a carbon content (0.85% > -1 .05% o) and a manganese content (16-19%) associated with other properties of the invention results in strength values greater than 1,200. M Pa and a product P
(strength x elongation at break) greater than 60,000, or even 65,000 MPa%. In Figure 1 it will be seen that these steel compositions are in a region where the SFE is about 19-24 mJ / m2, ie favorable for the maclage deformation. However, the inventors have also shown that a variation in the local content of carbon or manganese has a much lower influence than that mentioned in the previous example. This is because measurements of variations in local contents (CL, M nL) made in various austenitic steel compositions of Fe-CM n have shown, under identical manufacturing conditions, a carbon and manganese cosegregation very close to the one illustrated in figure 1. Under these conditions, a variation in the local content (CL, Mn) has only a small consequence in the mechanical behavior, since the segment that represents this cosegregation goes in a direction approximately parallel to the iso-SFE curves. further, the inventors have shown that the formation of martensite must be absolutely avoided during deformation operations or during the use of the sheet, for fear that the mechanical properties in the pieces are heterogeneous. The inventors have determined that this condition is satisfied when, at any point of the sheet, the local carbon and manganese contents of the sheet are such that:% M n + 9.7% C >; 21 .66. In this way, thanks to the characteristics of the nominal chemical composition defined by the invention, and those defi ned by the
local contents of carbon and manganese, an austenitic steel sheet is obtained that not only has very good mechanical properties, but also presents very low dispersion of these properties. A person skilled in the art, thanks to his general knowledge, will adapt the manufacturing conditions to satisfy this relationship with respect to the local content, in particular by means of the molding conditions (molding temperature, electromagnetic stirring of the liquid metal) or the conditions of reheating that results in the homogenization of carbon and manganese by diffusion. In particular, it will be advantageous to carry out processes for molding semi-finished products in the form of a thin slab (with a thickness of a few centimeters) or in the form of a thin strip, since these processes are generally associated with a reduction in local compositional heterogeneities. By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention Example: Steels were melted with the following nominal composition
(contents expressed in percentage by weight): Table 1: Nominal compositions of steels
After molding, a steel semi-finished product I according to the invention was reheated to a temperature of 1180 ° C and hot rolled to a temperature above 900 ° C to achieve a thickness of 3 mm. A retention time of 2 s was saved after rolling, to complete the recrystallization, and then the product was cooled at a rate greater than 20 ° C / s followed by winding at room temperature.
The reference steels were reheated to a temperature above 1150 ° C, rolled to a rolling end temperature higher than 940 ° C and then rolled at a temperature below 450 ° C. The recrystallized surface fraction was 100% for all steels, the fraction of precipitated carbides was 0% and the average particle size was between 9 and 10 μm. The tensile properties of the hot-rolled sheets were as follows: Table 2: Traction properties of Has sheets laminated hot @n
In comparison with the reference steel R1, whose mechanical properties are already good, the steel according to the invention made it possible to obtain an increased strength of approximately 200 MPa, with a very comparable elongation. To evaluate the structural and mechanical homogeneity during the deformation, similar concave shapes were produced, on which the microstructure was examined by X-ray diffraction. In the case of reference steel R2, the appearance of martensite was observed whenever the ratio of deformation exceeded 17%, resulting in the operation of total stretch fracture. An analysis indicated that the characteristic:% MnL + 9.7% C > 21.66 was not met at any point (figure 1). In the case of steel according to the invention, not even a small amount of martensite can be found, and a similar analysis indicated that the characteristic:% MnL + 9.7% > C > 21.66 was fulfilled at any point, thus preventing the appearance of martensite. The steel sheet according to the invention was then subjected to a slight cold deformation by rolling by an equivalent deformation of 14%. The strength of the product was then 1420 MPa and its elongation at break was 42%), that is, a product P = 59.640 MPa%. This product that had exceptionally good mechanical properties offers a great possibility of subsequent deformation due to its plasticity conservation and low anisotropy.
Furthermore, after the winding, unwinding and pickling steps, the hot-rolled steel sheet according to the invention and that of the steel R1 were then cold-rolled, before annealing to obtain a completely recrystallized structure. Average austenitic granulometry, strength and elongation at break are indicated in the following table. Table 3: Mechanical Properties of Two Cold Rolled and Annealed Walk Products
The steel sheet produced according to the invention, whose granulometry is 4 microns, therefore provides a particularly advantageous strength / elongation combination and a significant increase in strength compared to the reference steel. As in the case of hot-rolled sheet products, these properties are obtained with a very large homogeneity in the product, without even small amounts of martensite being present after deformation. The equibiaxial expansion tests using a punch
Hemispherical 75 mm diameter, made in a cold rolled and annealed sheet of 1, 6 mm thick, according to the invention, gave a depth of stretch limit of 33 mm, demonstrating excellent deformability. The curvature tests performed on this same sheet also showed that the critical deformation before cracks appeared was greater than 50%. The steel sheet produced according to the invention was subjected to cold deformation by rolling with an equivalent deformation ratio of 8% > . The product resistance was then 1420 M Pa and its elongation at break was 48%, that is, a product P = 68.160 M Pa%. In this way, due to their particularly good mechanical properties, their very homogeneous mechanical behavior and their microstructural stability, the hot-rolled or cold-rolled steels according to the invention will be advantageously used for applications where high deformability is desired. and a very high resistance. When used in the automotive industry, its advantages will be used beneficially for the manufacture of structural parts, reinforcement elements and even external parts.